1. Technical Field of the Invention
The present invention relates to radiofrequency amplification and more particularly, but not exclusively, to the radiofrequency stages of radiofrequency signal receivers, such as for example cellular mobile telephones operating according to such standards as GSM, CDMA, DCS, etc.
2. Description of Related Art
In radiofrequency receiver or transmitter architectures, the performance on the transconductance value is most often limited by the current flowing within the transconducting element, by its degeneracy and by its load impedance.
For example, in a dual-band receiver of the direct-conversion type, such a transconductor element is used in the low-noise amplifier (LNA) stage.
The performance on the transconductance value of the LNA is essential and conditions parameters such as the noise figure of the mixer situated downstream.
By way of example, if a bipolar transistor arranged in a common-emitter configuration and having a resistive load impedance R on its collector is taken as the transconductor element, it is shown that the transconductance value gm of such an amplifier stage is defined by the ratio of the collector dynamic current over the radiofrequency voltage received on the base of the transistor.
To a first approximation, this transconductance value gm is then equal to:
                              gm          T                ⁢                  1                      1            +                          j              ⁢                                                          ⁢              CR              ⁢                                                          ⁢              ω                                                          (        1        )            where gmT denotes the transconductance value of the transistor, C denotes the collector-base capacitance of the transistor, R the load impedance of the transistor and ω the angular radio frequency.
It can therefore be seen that the total transconductance gm is impacted by the transistor base-collector capacitance in addition to the resistive load impedance R.
The resistive load impedance is usually replaced by a cascode transistor connected to the collector of the LNA. The cascode transistor represents the best compromise between the transconductance and the noise figure. Such a solution is for example described in the article by Keng Leong Fong entitled “High-Frequency Analysis of Linearity Improvement Technique of Common-Emitter Transconductance Stage Using a Low-Frequency-Trap Network”, IEEE Journal of Solid-State Circuits, Vol. 35, No. 8, August 2000.
Indeed, aside from the fact that the cascode transistor increases the isolation between the LNA amplifier and the mixer situated downstream, it also allows a low dynamic impedance (equal to the inverse of the transconductance of the cascode transistor), compared with the aforementioned load resistive impedance, to be presented to the collector of the amplifier transistor.
One solution for improving the transconductance gm, when a differential configuration is employed, consists in providing capacitive cross-coupling between the LNA collector and the base or the gate of the cascode transistor. This configuration reduces the dynamic impedance presented by the cascode transistor by a factor of two.
Another solution, based on a positive feedback technique, described in the article by Francesco Centurelli et al. entitled “A Bootstrap Technique for Wideband Amplifiers'; IEEE Transactions on Circuits and Systems—I: Fundamental Theory and Applications”, Vol. 49, No. 10, October 2002, allows the pole introduced by the base-collector capacitance to be compensated by an anti-pole. This results in an increase in the bandwidth and in the radiofrequency transconductance of the amplifier. Nevertheless, this wideband solution requires a resistive degeneration for the amplifier, which necessarily leads to an increase in the noise figure.
A need accordingly exists to provide a different solution to the problem of improving the transconductance value of a radiofrequency amplifier stage.